<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .expand {
            height: 100px;
        }
    </style>
</head>

<body>
    <button class="btn">展开/收缩 </button>
    <button class="box">
        盒子
        </div>
</body>
<script>
    // const btnList = document.getElementsByTagName('button')
    const btnList = document.querySelectorAll('button')
    console.log(btnList);

    for (var i = 0; i < btnList.length; i++) {
        const element = btnList[i];
        element.addEventListener('click', () => {
            console.log(i)
            // document.body.appendChild(document.createElement('button'))
        })
    }
</script>

</html>